1. Field of the Invention
The present invention relates to a method and an apparatus for the nondestructive and contact-free detection of faults, particularly by means of eddy currents, in a test specimen which is moved relative to a probe that is characterized by an effective width.
2. Description of Related Art
A conventional measurement method for the nondestructive and contact-free detection of faults in a test specimen, in particular a metal semifinished product, is to induce and measure eddy currents in the test specimen. In this case, periodic electromagnetic alternating fields are applied to the test specimen using a transmission coil which is energized sinusoidally. The eddy currents which are thereby induced in the test specimen, in turn, induce a periodic electrical signal in a coil arrangement which is used as a probe and can have an individual coil (“absolute coil”) or two coils which are connected subtractively (“differential coil”), the electrical signal having a carrier oscillation corresponding to the transmitter carrier frequency, the amplitude and/or phase of which is/are modulated in a characteristic manner as a result of a fault in the test specimen if a fault reaches the sensitive region of the probe, i.e., the effective width of the probe. In order to scan the test specimen, the test specimen is usually moved linearly with respect to the probe, but arrangements having a rotating probe are also known. The signal detected by the probe is usually demodulated in an analog manner, for example, using synchronous demodulation, and is then evaluated in order to detect faults in the test specimen. In this case, the signal is usually digitized only for the evaluation and representation of the fault signal, that is to say after the coil signal has been demodulated.
Such eddy current measurement methods are relatively complicated and expensive on account of the outlay needed for the analog demodulation. It must also be taken into account that for different relative speeds between the test specimen and the probe, that is to say in the case of different output rates and test speeds, different sets of filters are usually required for the demodulated signal, thus entailing additional outlay in the case of a variable test specimen speed.
U.S. Pat. No. 5,175,498 describes an eddy current measurement method in which even the measurement signal which has been picked up by the coil probe is digitized using a triggerable A/D converter and is then filtered in digital form using Fourier transformation. Triggering of the A/D converter, i.e., the sampling rate, is controlled as a function of the forward feed speed (detected using an encoder) of the test specimen in order to avoid errors (which result from the test specimen being moved backward) when evaluating the signal.
U.S. Pat. No. 4,445,088 describes a stray magnetic flux measurement method in which a metal test specimen is moved relative to a probe, the measurement signal detected by the probe being digitized using a triggerable A/D converter after said signal has passed through a bandpass filter, and triggering of the converter, i.e., the sampling rate, being controlled by the forward feed speed (detected using a speed sensor) of the test specimen. In order to detect faults, the amplitude of the digitized signal is evaluated in order to determine whether it has exceeded a threshold value, the selection of the sampling rate as a function of the testing speed being used to achieve a prescribed measurement accuracy that is independent of the test specimen speed.